Phase Plate for a TEM

ABSTRACT

A phase plate, specifically a Zernike type phase plate, for use in an electron microscope, comprises a central hole, and a thin film causing a phase shift of the electrons passing through said film. This phase shift causes the Contrast Transfer Function (CTF) to change from a sine-like function to a cosine-like function. 
     The phase plate is equipped with a film in the form of an annulus, carried by a much thinner film. As a result only in a small spatial frequency range (for low frequencies) the phase is changed (and thus the CTF), and for other spatial frequencies the phase shift is negligible, and thus the CTF remains unchanged. Due to the much smaller thickness of the carrier film the scattering of electrons is negligible as well.

This Application claims priority from U.S. Provisional Application61/650,584, filed May 23, 2012, which is hereby incorporated byreference.

TECHNICAL FIELD OF INVENTION

The invention relates to a phase plate for an electron microscope, themicroscope generating a beam of electrons, said beam of electronspassing through a sample, the beam of electrons after passing throughthe sample divided in an undiffracted beam and a diffracted beam, thesample imaged in an image plane where the undiffracted and thediffracted beams interfere to form an image, the phase plate placed in aplane where the undiffracted and the diffracted beam are focused, thephase plate comprising:

-   A central area, said central area in working causing a first phase    shift,-   A first thin film surrounding and bordering the central area, said    first thin film in working causing a second phase shift differing    from the first phase shift,-   A holder structure surrounding the first thin film for holding the    phase plate, the difference between first phase shift and second    phase shift causing an improvement of the contrast in the image for    a range of spatial frequencies.

BACKGROUND OF THE INVENTION

Such a phase plate is known from “Phase Contrast Enhancement with PhasePlates in Biological Electron Microscopy”, K. Nagayama et al.,Microscopy Today, Vol. 18 No. 4, July 2010, pages 10-13, furtherreferred to as Nagayama [-1-], and is also known as a Zernike phaseplate.

In a Transmission Electron Microscope (TEM) a sample is imaged bypassing a beam of energetic electrons with a selectable energy of, forexample, between 40 keV and 400 keV, through the sample. For so-calledweak-phase samples, such as biological samples, most electrons passthrough the sample while some electrons are elastically or inelasticallyscattered, the elastically scattered electrons forming diffracted beams.The image is formed by interference of the elastically scattered andunscattered electrons (diffracted and undiffracted beams).

A problem arises in that the Contrast Transfer Function (CTF) for lowspatial frequencies in the image is zero or close to zero, resulting inlow visibility of large objects/structures. This is caused by the factthat a camera or fluorescent screen at the image plane is sensitive tointensity variations, but not to phase variations of the impingingelectron beam.

SUMMARY OF THE INVENTION

A solution to this is the use of a phase plate, the phase plateintroducing a phase difference between the diffracted beams and theundiffracted beam. There are two main types of phase plates: theso-called Zernike phase plate leaves the undiffracted beam unchanged,and causes a phase shift of the diffracted beams, while the so-calledBoersch phase plate shifts the undiffracted beam while leaving thediffracted beams unchanged.

The introduction of the phase difference by the phase plate changes thesine-like behavior of the CTF to a cosine-like behavior, and thus amaximum contrast for low spatial frequencies. The invention describedhere relates to a Zernike phase plate as described by Nagayama [-1-],more specifically in its FIG. 5. The phase plate is positioned in thediffraction plane of the objective lens, also known as the back-focalplane of this lens. In this plane the parallel beam impinging on thesample, and the parallel diffracted beams originating from the sample,are focused into spots. The central spot in this plane is passed througha central through-hole in the phase plate, and thus experiences no phaseshift. Electrons passing outside the hole pass through a foil andexperience a phase shift φ of preferable −π/2. The interference of theundiffracted and the diffracted beams now results in intensitymodulation instead of phase modulation.

It is noted that the phase plate as described is mounted in thediffraction plane of the TEM, but that the phase plate can be mounted inany plane downstream of the diffraction plane that is an image planethereof.

It is further noted that the phase plate can be a coated phase plate asshown in Nagayama [-1-], FIG. 5, but that coating is not essential: aslong as the electrons pass through sufficient material to experience aphase shift with respect to the electrons passing through the centralarea or through-hole, phase modulation is translated in intensitymodulation.

It is also noted that, although a phase shift φ of −π/2 is preferred,for other phase shifts the phase modulation is at least in parttranslated in an intensity modulation.

A problem with the known phase plate is that part of the electronspassing through the foil are scattered by the foil and do not contributeto the image formation in the image plane of the sample. This ismentioned in e.g. “Optimizing phase contrast in transmission electronmicroscopy with an electrostatic (Boersch) phase plate”, E. Majorovitset al., Ultramicroscopy Vol. 107 (2007), pages 213-226, further referredto as Majorowits [-2-]. At page 215, left column, it mentions as anadvantage of the Boersch type phase plate over the Zernike type phaseplate: “The advantage of the Boersch phase plate is obvious: no furtherelectron scattering is introduced into the beam path, which avoidspossible contamination problems and additional inelastic or pluralscattering. High-resolution information is not affected and istransferred in the phase contrast image at its full signal strength.”

Another problem is that, although at another spatial frequency, the CTFshows zeros, and thus spatial frequency bands with low contrast,resulting in low visibility of objects/structures with correspondingdimensions.

The invention aims to provide at least a partial solution for one orboth of these problems.

To that end a phase plate according to the invention is characterized inthat between the first thin film and the holder structure at least onefurther thin film is present, said further thin film in working causinga further phase shift differing from the second phase shift, thedifference between first phase shift and further phase shift for afurther range of spatial frequencies causing an improvement of thecontrast in the image compared to the situation where the further phaseshift would equal the second phase shift.

The invention is based on the insight that for a first part of the CTF,corresponding to low spatial frequencies, it is beneficial to introducea phase shift of approximately −π/2, thus changing the CTF from asine-like behavior to a cosine-like behavior, but when for higherspatial frequencies the resulting absolute value of the CTF, |CTF|, isless than the |CTF| achieved without phase plate, it would have beenbetter to do without this phase shift.

It is noted that the CTF can be written as

CTF(k)=A(k)E(k)sin(χ(k)+φ)  [1]

where k is the spatial frequency, A(k) is the aperture function, E(k)the attenuation of the wave for higher spatial frequencies k, alsocalled envelope function, χ(k) a function of the aberrations of theelectron optical system and φ the phase shift introduced by a phaseplate (if present). The value of A(k) and E(k) are always less thanunity, and thus the maximum attainable is a function of the spatialfrequency k and less than one. As the maximum value of sin(χ(k)+φ) for aproper φ equals 1, the maximum attainable contrast transfer function isdenoted by

CTF_(m)(k)=A(k)E(k)   [2]

The improved phase plate described here preferably comprises a firstthin film preferably formed as a first annulus causing a second phaseshift and a further thin film preferably formed as an annulussurrounding and bordering the first annular area causing a further phaseshift different from the second phase shift. This enables tuning the CTFfor different spatial frequencies with different phase shifts.

Preferably the CTF caused by the outer edge of the first thin filmequals the CTF of the inner edge of the further thin film, therebyjoining the CTF for both films without discontinuity.

In a preferred embodiment the central area of the phase plate is athrough-hole.

Although strictly only a phase difference between the central,undiffracted beam and the diffracted beam is required, preferably thecentral beam passes through a through-hole in the film, thereby avoidingelectrons from the intense central beam to impinge on the foil and causeproblems like heating, contamination, charging and the like.

Preferably the first phase shift caused by the first thin film isapproximately −π/2, although small deviations do not change the CTF forlow spatial frequencies much: a deviation of, for example, π/10,resulting in a phase shift of (−π/2±π/10) gives an almost identicalcontrast for low spatial frequencies.

Preferably the first thin film changes the |CTF| such, that|CTF|≧0.5×CTF_(m) for electrons impinging on this first thin film,whereas the |CTF| without the phase plate would be below 0.5×CTF_(m).

Likewise the further thin film preferably causes a phase shift such thatthe |CTF| is kept above 0.5×CTF_(m) for electrons impinging on thisfurther thin film, where the phase plate described by Nagayama [-1-]would give a |CTF_(m)| below 0.5×CTF_(m) for said range of frequencies.More preferably the first and the further thin film should cause aphaseshifts resulting in a |CTF|≧0.7×CTF_(m) for electrons impinging on thesethin films.

The skilled artisan will recognize that a graphene layer or a graphenedouble layer is preferred as further thin layer.

Preferably the further thin film improves the |CTF| for the electronsimpinging on said further thin film from a value of less than0.5×CTF_(m)(k) to a value in excess of 0.5×CTF_(m)(k) for electronsimpinging on this further thin film, at least at the position wherefirst thin film and second thin film border, more specifically from avalue of less than 0.7×CTF_(m)(k) to a value of at least 0.7×CTF_(m).

For electrons impinging further from the central beam (from the axis),the electrons fall on the thinner film, and hardly experience any phaseshift or scattering. This is thus a situation that corresponds with thefirst maximum of the |CTF| in microscopes without a conventional phaseplate. Therefore the phase plate according to the invention shows afirst spatial frequency range where the sine-like function is phaseshifted to a cosine-like function, and a further spatial frequency rangewhere the CTF is (almost) identical to the CTF of a microscope withoutphase plate.

The phase plate according to the invention thus has an improved |CTF|over a microscope equipped with a conventional phase plate and amicroscope without a phase plate. Compared to a microscope with aconventional phase plate the scattering of electrons out of the beam bythe phase plate is also less, resulting in more electrons contributingto the image.

It is noted that more thin films, preferably in the form of concentricannuli, may be used. The phase shift of these annuli may differ fromeach other, either by using different materials or by using differentthicknesses. In this way a phase plate resembling a Zone plate may beconstructed, where correction of the CTF for numerous frequencyintervals may be used.

It is further noted that in “Practical factors affecting the performanceof a thin-film phase plate for transmission electron microscopy”, Danev,R. et al, Ultramicroscopy 109 (2009), pages 312-325 [-4-], morespecifically its FIG. 1, a phase plate is displayed, consisting of athin film with a central hole. Electrons passing through the centralhole will experience no phase shift, and electrons passing through thethin film a phase shift. In said publication a contamination is shown onthe thin film. Locally the phase plate will thus show a phase shiftdifferent from the phase shift in the rest of the thin film. However,this contaminant does not form a concentric annulus.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is now elucidated using figures, where identical referencenumerals indicate corresponding features. To that end:

FIG. 1 schematically shows a TEM equipped with a phase plate

FIG. 2 ^(a) schematically shows a prior art phase plate, and

FIG. 2 ^(b) schematically shows the corresponding CTF, compared to theCTF of a conventional TEM without phase plate;

FIG. 3 ^(a) schematically shows a first embodiment of a phase plateaccording to the invention, and

FIG. 3 ^(b) schematically shows the corresponding CTF;

FIG. 4 ^(a) schematically shows a second embodiment of a phase plateaccording to the invention, and

FIG. 4 ^(b) schematically shows the corresponding CTF;

FIG. 5 schematically shows a third embodiment of a phase plate

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 schematically shows a TEM 100 equipped with a phase plate.

FIG. 1 shows a particle source 104 producing a beam of particles, suchas electrons, along optical axis 102. The particles have a selectableenergy of typically between 80-300 keV, although higher energies, e.g.400 keV-1 MeV, or lower energies, e.g. 50 keV, may be used. The beam ofparticles is manipulated by condenser system 106 to form a parallel beamimpinging on a sample 108, the sample positioned with a sample holder110. The sample holder can position the sample with respect to theoptical axis and may shift the sample in the plane perpendicular to theoptical axis and tilt the sample with respect to said axis. Objectivelens 112 forms a magnified image of the sample. The objective lens isfollowed by a magnifying system 116, e.g. a doublet of lenses, formingan enlarged image of the back-focal plane 114 of the objective lens. Aphase plate 118 is placed in an enlarged image of the back-focal planeof the objective lens, this conjugated plane positioned between themagnifying system and a projection system 122. The magnifying system 122thus forms an image of the back-focal plane with a variablemagnification. The phase plate is positioned with a manipulator 120,allowing the phase plate to be centered round the optical axis. Theprojection system forms a magnified image of the sample on a detector124, thereby revealing sample details of e.g. 0.1 nm. The detector maytake the form of a fluorescent screen, or e.g. a CCD camera. In the caseof e.g. a fluorescent screen the screen can be viewed via the glasswindow 126.

To align the optical components on the axis the TEM comprises a largenumber of deflectors, schematically shown as 128-1 . . . 128-7, althoughother deflectors on other places may be included.

It is noted that the phase plate may also be positioned in theback-focal plane itself.

FIG. 2 ^(a) schematically shows a prior art phase plate, known from e.g.Nagayama [-1-]. It shows a holder structure 20 in the form of, forexample, a standard platinum diaphragm with an aperture, as routinelyused in electron microscopes. The outer diameter of such a standardplatinum diaphragm is typically 3.05 mm, although other diameters areused. On this holder structure a thin foil 22 of amorphous carbon isplaced. As such foils are often contaminated with charging particles orsuch like (not shown), one of the last fabrication steps involvescovering the layer 24 with a carbon coating. The carbon coating istypically made by vacuum evaporation, resulting in an additionalamorphous carbon layer.

The centre of the foil shows a through-hole 26 for passing theundiffracted beam. This hole has a diameter sufficient to pass theundiffracted beam, but preferably not much more, as then slightlyscattered electrons, corresponding with electrons scattered by largestructures (low spatial frequencies) do not pass through the foil to getthe required phase shift but instead through the central hole. Typicallythe through-hole shows a diameter of less than 1 μm, formed in the foilusing a focused ion beam.

The phase plate preferable shows rotational symmetry around axis 28.

The electrons passing through the carbon exhibit a phase shift as aresult of the internal potential of the carbon. The relation of thethickness T of the amorphous carbon film(s) and the phase shift φ causedby passing through said film is described by Motoki [-3-]:

φ=−σV _(av) T   [1^(a)]

σ=e/(h-V _(e))   [1^(b)]

where V_(av) is the averaged inner potential of amorphous carbon, e isthe elementary electric charge of an electron, h- is Plank's constantand V_(e) is the velocity of an incident electron. The averaged innerpotential of amorphous carbon is 7.8 V and σ is 0.00892 (V·nm)⁻¹ for 300keV electrons.

With these values, the thickness T of a carbon phase plate is calculatedto be 31 nm for a phase shift φ=−π/2 (for 300 keV electrons).

It is noted that this prior art phase plate uses a round diaphragm asholder structure (support structure), but that also silicon structures(with a round or square inner or outer perimeter) are known to be used.Such a silicon phase plate can be made using lithographic techniques,optionally combined with ion beam milling for forming the centralthrough-hole in the thin film. It is further noted that traditionallycarbon is used for the thin film of prior art phase plates. However,other materials such as silicon may be used.

An overview of inner potentials and foil thicknesses for carbon, siliconand silicon nitride for different electron beam energies is given intable 1.

TABLE 1 calculated film thickness to achieve φ = −π/2 for a Zernikephase plate as a function of beam energy for different materials. Filmthickness [nm] for φ = −π/2 Material V_(av) [V] 100 keV 200 keV 300 keVC 7.8 21.8 27.6 30.9 Si 12.6 13.5 17.1 19.1 SiN 12.6 13.5 17.1 19.1

FIG. 2 ^(b) schematically shows the CTF of the prior art Zernike phaseplate shown in FIG. 2 ^(a), compared to the CTF obtained without phaseplate.

In FIG. 2 ^(b) the CTF is shown in the so-called Scherzer defocus. Theaperture function as mentioned in formula -1- is represented by curve204, the envelope function E(k) is represented by curve 206. The product208 of these two, the CTF_(m)(k), is typically almost identical to theenvelop function. Without phase shift (without phase plate), so withφ=0, the resultant CTF is represented by the sine-like function 200.With a phase shift of −π/2 the CTF changes to the cosine-like function202. The result is that the introduction of phase shift enables imaginglarge structures (low spatial frequencies). However, zero crossings willoccur at higher spatial frequencies in both cases, although the zerocrossings occur at different spatial frequencies.

A disadvantage of this type of phase plate is that electrons scatteredby the sample must pass through the carbon film, said carbon film againscattering part of the electrons from the beam, resulting in the loss ofinformation, and thus an increase of the signal-to-noise ratio in theimage. Also, the introduction of the phase plate introduces zeros wherethe absolute value of the CTF without phase plate has a maximum, and atthese spatial frequencies thus decreases the visibility of thecorresponding structures compared to a situation where no phase platewould have been used.

It is noted that typically approximately 20% of the electrons impingingon foil 22 are scattered from the beam, and thus lost for imaging.

FIG. 3 ^(a) schematically shows a first embodiment of a phase plateaccording to the invention.

The phase plate shows a similar holder structure 20 as the prior artphase plate. A thin film 30 is mounted on the holder, the thin layerhaving little effect on the beam. Inside diameter D₁ an extra layer 32is added to the thin layer, thus forming a first thin film with addedthickness, whereby the part of the phase plate inside D₁ resembles theprior art phase plate, showing a thin film that causes a phase shift of−π/2, but between D₁ and the inner perimeter of the holder the film ismuch thinner and the phase shift much smaller than in prior art phaseplates. The result is that electrons passing through the film 32 areshifted over −π/2, but electrons passing though the phase plate outsideD₁ are shifted much less, for example over −π/10 (for 300 keVcorresponding to a carbon layer of approximately 6 nm). Therefore theCTF for these electrons resembles the CTF of a conventional microscopewithout phase plate, and as the film is thinner than in prior art phaseplates, also the scattering by said film is less.

FIG. 3 ^(b) shows the CTF of said phase plate. The CTF of this phaseplate is given by curve 300. It shows a first region at low spatialfrequencies where the CTF is modified from a sine-like behavior to acosine-like behavior, similar to curve 202 shown in FIG. 2 ^(b).Following this region is a second region where it equals the CTF of aconventional microscope without phase plate, similar to curve 200 shownin FIG. 2 ^(b). the two regions are stitched together at point 302,corresponding with a diameter D₁ of the film shown in FIG. 3 ^(a),chosen such that the two regions show continuity.

It is noted that it is possible to have continuity in |CTF| withouthaving continuity in CTF: this implies that the contrast changes fromblack/white to white/black instantaneously. This is in most cases anunwanted situation.

It is further noted that for optimum performance (read: |CTF|) it may beattractive to chose another defocus than Scherzer defocus: the intent isto have as large a region as possible where the CTF is far removed fromzero.

FIG. 4 ^(a) shows another embodiment of the phase plate according to theinvention, in which another thin film ring surrounds the further thinfilm.

This embodiment shows a phase plate comprising a further thin filmformed as a first annulus 32 as described in the first embodiment,augmented with another annulus 34 surrounding the first annulus. Theannuli 32 and 34 may have the same thickness, or different thicknesses(as shown). The result of this embodiment is that for a first range ofspatial frequencies, corresponding with all electrons falling on thephase plate within diameter D₁ (but outside of the central hole) a firstphase shift φ₁ is achieved, for a second range of spatial frequenciescorresponding with electrons falling on the phase plate betweendiameters D₁ and D₂, a phase shift φ₂≈0, and a third range of spatialfrequencies corresponding with electrons falling on the phase plateoutside diameter D₂, a phase shift φ₃. In this way the CTF can be tunedfor optimal contrast for a large range of spatial frequencies.

It is noted that the annulus 34 shown is separated from the annulus 32,but the annuli 32 and 34 may also touch each other.

As the person skilled in the art will recognize, it is possible to forma phase plate comprising a number of annuli, thus showing resemblancewith a zone plate as used in e.g. x-ray optics. Also modulation of thethickness of the annulus or annuli in a discrete or non-discrete fashionis possible. However, as the CTF is also a function of the defocus, toomany annuli or thickness modulation over a too large diameter may becomeimpractical.

FIG. 4 ^(b) schematically shows the CTF of the phase plate shown in FIG.4 a.

It shows a first region at low frequencies, joined to a second regionbetween the points 402 and 404, glued to yet another region from 404 tohigher frequencies.

FIG. 5 shows a third embodiment of the phase plate according to theinvention. In this embodiment the scattering of the further thin film isfurther diminished by forming the film as a number of spokes 50. Thefirst thin film is now only partly surrounded by the further thin film,but most of the first thin film is surrounded by empty spaces 52.Electrons passing through these empty spaces are not scattered, and onlypart of the electrons impinging on the spokes are scattered. The phasedifference of the electrons passing through the empty space and thespokes is small, and therefore the CTF of electrons falling on thespokes and those passing between the spokes is minimal. Therefore theimage reconstructed by the interference of electrons in the image plane(at the detector or fluorescent screen) will be isotropic: it does notor hardly show an angular dependency in CTF caused by the phase plate.

It is noted that the scattering of electrons by the thin film does notlead to a different CTF, but in a decrease in signal-to-noise. Thisimplies that more signal (impinging electrons) should be used to obtainan image with the same signal-to-noise. As known to the skilled artisanthis may lead to damage of the sample, and thus to a lower quality ofthe images.

LITERATURE

-   [-1-] “Phase Contrast Enhancement with Phase Plates in Biological    Electron Microscopy”, K.

Nagayama et al., Microscopy Today, Vol. 18 No. 4, July 2010, pages10-13.

-   [-2-] “Optimizing phase contrast in transmission electron microscopy    with an electrostatic (Boersch) phase plate”, E. Majorovits et al.,    Ultramicroscopy Vol. 107 (2007), pages 213-226.-   [-3-] “200 kV TEM with a Zernike phase plate”, S. Motoki et al.,    Microsc. Microanal. 11 (Suppl 2), 2005, pages 708-709.-   [-4-] “Practical factors affecting the performance of a thin-film    phase plate for transmission electron microscopy”, Danev, R. et al,    Ultramicroscopy 109 (2009), pages 312-325.

1. Phase plate suitable for an electron microscope, the microscopeadapted to generate a beam of electrons, said beam of electrons inworking passing through a sample, said beam of electrons in working,after passing through the sample, divided in an undiffracted beam and adiffracted beam, the sample in working imaged in an image plane wherethe undiffracted and the diffracted beams in working interfere to forman image, the phase plate placed in a plane where the undiffracted andthe diffracted beam in working are focused, the phase plate comprising:a central area, said central area in working characterized by a firstphase shift, a first thin film surrounding and bordering the centralarea, said first thin film in working characterized by a second phaseshift differing from the first phase shift, a holder structuresurrounding the first thin film suitable for holding the phase plate,thedifference between first phase shift and second phase shift in workingcausing an improvement of the contrast in the image for a range ofspatial frequencies, wherein between the first thin film and the holderstructure at least one further thin film is present, said further thinfilm in working characterized by a further phase shift differing fromthe second phase shift, the difference between first phase shift andfurther phase shift for a further range of spatial frequencies inworking causing an improvement of the contrast in the image compared tothe situation where the further phase shift would equal the second phaseshift the first thin film and the further thin film are formed asconcentric annuli, the annulus forming the further thin film surroundingand bordering the annulus forming the first thin film, the centre of theannuli coinciding with the middle of the central area
 2. The phase plateof claim 1 in which the central area is a through-hole, and the firstphase shift thus equals zero.
 3. The phase plate of claim 1 in which thefirst thin film in working causes a phase shift such, that the absolutevalue of the Contrast Transfer Function, |CTF| of the correspondingfrequency range is changed from a value of at most 0.5 of the maximumattainable CTF to a value of at least 0.5 of the maximum attainable CTF,more specifically from a value of less than 0.7 of the maximumattainable CTF to a value of at least 0.7 of the maximum attainable CTF.4. The phase plate of claim 1 in which the further thin film in workingcauses a phase shift such, that in said further range of spatialfrequencies |CTF| of the corresponding frequency range is changed from avalue of at most 0.5 of the maximum attainable CTF to a value of atleast 0.5 of the maximum attainable CTF, more specifically from a valueof less than 0.7 of the maximum attainable CTF to a value of at least0.7 of the maximum attainable CTF.
 5. The phase plate of claim 1 inwhich the further thin layer is a graphene layer or a graphenedouble-layer.
 6. The phase plate of claim 1 in which the thickness of atleast one of the thin films is modulated as a function of the distancefrom the center in such a way that in working the resultant absolutevalue of the |CTF| is for the corresponding frequency range above 0.5 ofthe maximum attainable CTF, more specifically more than 0.7 of themaximum attainable CTF, most specifically more than 0.9 of the maximumattainable CTF.
 7. The phase plate of claim 6 in which the CTF inworking does not change sign for the frequency corresponding to theouter edge of the first thin film and the inner edge of the further thinfilm.
 8. The phase plate of claim 1 in which the first thin film and thefurther film show a difference in thickness, as a result of which inworking the phase shift caused by the first thin film and the furtherthin film differ.
 9. The phase plate of claim 1 in which the first thinfilm and the further thin film show materials with different averagedinternal potentials, as a result of which in working the phase shiftcaused by the first thin film and the further thin film differ.
 10. Thephase plate of claim 1 in which the further thin film shows openings,the first thin layer connected to the holder structure by spokesdividing the openings.
 11. An electron microscope, comprising: a sourceof electrons; an objective lens for focusing the electrons; a phaseplate in accordance with claim 1; an imaging device for forming an imagefrom the electrons.
 12. A method of imaging a sample in a chargedparticle beam system, the charged particle beam system, the methodcomprising: directing a beam of electrons through a sample, the passagedeflecting some of the electrons in the beam; passing the beam ofelectrons from the sample through a phase plate having three regions,each of the three regions having a different effect on the phase of theelectrons passing through the region; and combining the electrons thatpass through the three regions to form an image having improvedcontrast.
 13. The method of claim 12 in which passing the beam ofelectrons from the sample through a phase plate comprises passing thebeam of electrons through a phase plate that includes: a central areacausing the first phase shift; a first thin film surrounding andbordering the central area, the first thin film causing the second phaseshift; a holder structure surrounding the first thin film for holdingthe phase plate; and at least one further thin film between the firstthin film and the holder structure, the at least one further thin filmcausing the further phase shift; wherein the first thin film and the atleast one further thin film are formed as concentric annuli, the annulusforming the further thin film surrounding and bordering the annulusforming the first thin film, the centre of the annuli coinciding withthe middle of the central area.
 14. The method of claim 13 in which thecentral area is a through-hole and the first phase shift is equal tozero.
 15. The method of claim 13 in which the first thin film causes aphase shift such, that the absolute value of the Contrast TransferFunction, |CTF| of the corresponding frequency range is changed from avalue of at most 0.5 of the maximum attainable CTF to a value of atleast 0.5 of the maximum attainable CTF, more specifically from a valueof less than 0.7 of the maximum attainable CTF to a value of at least0.7 of the maximum attainable CTF.
 16. The method of claim of claim 13in which the further thin film causes a phase shift such, that in saidfurther range of spatial frequencies |CTF| of the correspondingfrequency range is changed from a value of at most 0.5 of the maximumattainable CTF to a value of at least 0.5 of the maximum attainable CTF,more specifically from a value of less than 0.7 of the maximumattainable CTF to a value of at least 0.7 of the maximum attainable CTF17. The method of claim 13 in which the further thin layer is a graphenelayer or a graphene double-layer.
 18. The method of claim 13 in whichthe thickness of at least one of the thin films is modulated as afunction of the distance from the center in such a way that theresultant absolute value of the |CTF| is for the corresponding frequencyrange above 0.5 of the maximum attainable CTF, more specifically morethan 0.7 of the maximum attainable CTF, most specifically more than 0.9of the maximum attainable CTF.
 19. The method of claim 18 in which theCTF does not change sign for the frequency corresponding to the outeredge of the first thin film and the inner edge of the further thin film.20. The method of claim 13 in which the phase shift caused by the firstthin film and the further thin film differ because the first thin filmand the further film show a difference in thickness or show materialswith different averaged internal potentials.
 21. The method of claim 13in which the further thin film shows openings, the first thin layerconnected to the holder structure by spokes dividing the openings. 22.The method of claim 12 in which passing the electrons through the phaseplate a phase plate having three regions causes a first phase shift, asecond phase shift, and a further phase shift, the difference betweenthe first phase shift and the second phase shift causing an improvementof the contrast in the image for a range of spatial frequencies, thedifference between the first phase shift and the further phase shiftcausing an improvement of the contrast in the image compared to thesituation where the further phase shift would equal the second phaseshift.